Silicates are the predominant material of the earth's crust. In neutral water the silicates are soluble to the extent of 70-120 ppm and are in equilibrium with silicic acid Si(OH).sub.4, a monomeric form of silica. Silicic acid is involved in both mineralization and biomineralization of complex structures. The fabrication of silicate structures is important in both ceramic and glass technology, but traditional fabrication technologies involve the use of high temperatures.
More recently, an alternative technology, referred to as sol-gel processing, has been developed which involves the formation and condensation of silicic acid under low temperature, aqueous conditions, which may be likened to mineralization and biomineralization. Sol-gel is a method for preparing specialty metal oxide glasses and ceramics by hydrolyzing a chemical precursor or mixture of chemical precursors that pass sequentially through a solution state and a gel state before being dehydrated to a glass or ceramic. Currently, sol-gel technology entails the deposition of silicon dioxide (or other metallic or non-metallic oxides) from supersaturated solutions, usually formed by hydrolysis of reactive precursors (alkoxysilanes or other metallic or non-metallic alkoxides).
Sol-gel technology has expanded dramatically since 1980 with the development of a variety of techniques to prepare fibers, microspheres, thin films, find powders and monoliths. Applications for sol-gel technology include protective coatings, catalysts, piezoelectric devices, wave-guides, lenses, high strength ceramics, superconductors, insulating materials and nuclear waste encapsulation. The flexibility of sol-gel technology allows unique access to multicomponent oxide systems and low temperature process regimens.
Preparation of metal oxides by the sol-gel route proceeds through three basic steps: 1) partial hydrolysis of metal alkoxides to form reactive monomers; 2) the polycondensation of these monomers to form colloid-like oligomers (sol formation); 3) additional hydrolysis to promote polymerization and cross-linking leading to a 3-dimensional matrix (gel formation). Although presented sequentially, these reactions occur simultaneously after the initial processing stage. ##STR2## As polymerization and cross-linking progress, the viscosity of the sol gradually increases until the sol-gel transition point is reached. At this point the viscosity abruptly increases and gelation occurs. Further increases in cross-linking are promoted by drying and other dehydration methods. Maximum density is achieved in a process called densification in which the isolated gel is heated above its glass transition temperature. The densification rate and transition (sintering) temperature are influenced primarily by the morphology and composition of the gel.
The sol-gel process allows the preparation of gels (amorphous or nearly amorphous, optically-clear, homogeneous solid) under various physical forms, including (1) fine, uniform and very reactive powders from rapid hydrolysis and drying at low temperature (less than 90.degree. C.); (2) optically clear, monolithic pieces from a slow hydrolyis route in a shaped container; or (3) films or fibers directly prepared from the viscous state of the alkoxide solution, or grown inside the solution. For a good description of sol-gel processing, see Ph. Colomban, "Gel Technology in Ceramics, Glass-Ceramics and Ceramic-Ceramic Composites," Ceramics International 15:23-50 (1989) .
The synthesis of light-weight silicate-based ceramic structures by sol-gel techniques is of enormous current interest. However, these sol-gel systems are unstable, difficult to control, and usually contain by-products which must be removed from the matrix. In order for sol-gel processing to achieve its full potential in the production of silicates, stress-free, low-shrinkage gels must be produced from highly concentrated silicate solutions containing minimum amounts of by-products. Therefore, it would be advantageous to be able to prepare an analog of silicic acid which has greater stability in aqueous solutions than silicic acid and allows greater synthetic control in the preparation of silicate structures.